1. Field of the Invention
The invention relates to a four-stroke internal combustion engine, especially for lawnmowers, comprising at least one cylinder for a reciprocating piston which acts via a connecting rod on a crankshaft and at least one overhead camshaft for driving overhead gas exchange valves and a transmission between crankshaft and camshaft with a motion plane within the transmission chamber which is perpendicular to the crankshaft axis.
2. The Prior Art
For the purpose of precise control of the valve opening times, overhead camshafts are increasingly used even in small driving engines such as lawnmowers for example, which means camshafts which are rotatably held in the cylinder head and are used for the direct or indirect actuation of the overhead intake and exhaust valves. A transmission between camshaft and crankshaft supplies the required driving power and simultaneously ensures the required synchronization of the camshaft rotation wit the crankshaft rotation. The working shaft driven by the engine generally has a vertical orientation in a number of areas of application of internal combustion engines. One example is the lawnmower with rotating blades. The vertical orientation is not kept in all cases however. Depending on the inclination of the base there is a respective inclination of the working shaft. In view of the simplest possible configuration and high efficiency by avoiding frictional losses, a direct coupling of the crankshaft of the driving engine with the working shaft by optional interposing of a simple spur gear is very advantageous. The crankshaft then also assumes a vertical orientation and, depending on the inclination of the base, an orientation more or less inclined towards the vertical.
The at least one “overhead” camshaft extends in such a configuration laterally and axially parallel to the camshaft axis. Continuous lubrication of the camshaft must be ensured in order to ensure reliable operation, especially under high operating temperatures and high speeds.
An internal combustion engine is known from DE 40 15 610 A1 in which an oil pump in the region of the lower bearing point of the crankshaft ensures a force-feed lubrication both of the especially stressed crankshaft con-rod bearing as well as the upper bearing point of the camshaft held in the cylinder head, therefore the overhead camshaft. The oil pump is connected for this purpose to a through port of the crankshaft, which both comprises an outlet towards the con-rod bearing as well as also converges in the region of the upper crankshaft end into cooling conduits of a hollow disk flywheel flanged on the crankshaft. The cooling conduits open into a cooling conduit within the hollow cylinder wall enclosing the piston, which cooling conduit extends parallel to the cylinder axis. Said cooling conduit comprises both passages which open into the crankshaft chamber as well as a passage to the upper camshaft. The oil emerging from this bearing can flow downwardly along the camshaft in order to both lubricate the camshafts as well as transmission between camshaft and crankshaft formed by a belt drive. The lower camshaft bearing can also thus be provided with sufficient lubricating oil. The back-flowing lubricating oil is collected in a separate, obviously lower-lying oil collecting container and is supplied to the oil pump again. The force-feed lubrication of this internal combustion engine requires a high constructional effort. The camshaft shown in DE 40 15 610 A1 is held at its ends, thus having a disadvantageous effect on the flexural strength of the camshaft.
An internal combustion engine with an overhead camshaft and vertically oriented crankshaft is known from EP 0 487 960 A1, in which the transmission formed by a belt drive lies at the upper end of the crankshaft and the camshaft. A force-feed lubrication is provided for the upper crankshaft bearing and the upper camshaft bearing. Oil dripping from the upper camshaft bearing lubricates both the cam surfaces as well as the lower camshaft bearing, from where it flows back to an oil storage annulus enclosing the crankshaft and receiving the oil pump. The transmission per se does not come into contact with the lubricating oil.
An internal combustion engine for a lawnmower is known from U.S. Pat. No. 5,000,126 in which a camshaft is used which is not overhead but is held in the crankshaft chamber.
It is the object of the present invention to provide an internal combustion engine which has a simple configuration, can be assembled with the lowest possible effort and ensures a reliable lubrication of the camshaft held in a deflection-resistant way in the operating states that may occur.